A technique of data processing as aforesaid has been described inter alia in PCT/GB94/00080, in which a series of data values are subjected to a so-called Datasieve, in which the values are successively filtered using a progressively increasing size criterion and the output of each successive filtering stage supplies the input to the next.
The Datasieve as described in PCT/GB94/00080 employs ordinal value filtering in each stage, and a commonly used ordinal value filter is a median filter.
This type of processing often requires a root signal to be obtained from each filtering stage (ie a signal that is invariant to further processing if recycled through the filter which has produced it). Using conventional ordinal value filters and particularly median filters, has involved the recycling of the filtered values two or three times or more through the same stage before a root signal has been obtained, and where this has to occur a number of times at each stage, the computational complexity and buffer storage requirements have detracted from the advantages obtained from Datasieve.
It is an object of the present invention to provide a filtering technique and apparatus for use in the successive filtering stages of the Datasieve which does not suffer from this particular disadvantage.
It is another object of the present invention to provide a method of operating the Datasieve at a higher speed.